This invention relates to a process for preparing polymer polyols via a vinyl polymerization process.
Polymer polyols are well known products which are used in the preparation of polyurethanes. The polymer polyols of interest herein are polymers of ethylenically unsaturated monomers which are polymerized in situ in a material having a plurality of active hydrogen-containing groups. Such products and processes for their preparation are described, for example, in U.S. Pat. Nos. 4,172,825, 4,233,425, 4,242,249, 4,287,323, 4,320,221, 4,350,780, 4,390,645, 4,394,491, 4,451,255, 4,460,715, 4,574,137, 4,661,531, 4,689,354, 4,745,153 and RE 32,733.
The most significant technical problem in making polymer polyols is that of forming a stable dispersion of particles having a desired particle size. The polymer particles tend to settle out of the continuous phase or to agglomerate. Often, low molecular weight oligomers form and remain dissolved in the continuous phase. These oligomers greatly increase the viscosity of the polymer polyol.
To alleviate this problem, most processes now employ a "stabilizer" or "dispersant", which according to theory becomes situated on the surface of the polymer particles and makes them more compatible with the continuous polyol phase. This stabilizer material is usually formed from a material such as a polyol which is compatible with the continuous phase, and which has been modified to contain induced unsaturation. The stabilizer is believed to copolymerize with the vinyl monomers so that the resulting polymer contains side chains which are soluble in the continuous phase, and thus help stabilize the particle against agglomeration and settling.
Several types of stabilizers are known, but the most effective ones previously known are or are derived from polyethers which are end-capped to introduce terminal polymerizable unsaturation. These capped polyethers may be homopolymerized or copolymerized with monomers such as styrene and/or acrylonitrile to form the actual stabilizer.
Although good results have been obtained with previous stabilizers, especially in conjunction with other process refinements, further improvements are still desired. Quite often, the stabilizer must be used in rather substantial quantities. Since the stabilizer is prepared from rather expensive raw materials, the use of such large quantities is economically disadvantageous. The stabilizer itself is an additional raw material which must be manufactured and stored, so it is desired for that reason as well to minimize its use. Beyond that, these previously known stabilizers are not as effective as desired. It is still desirable to further improve the stability and particle size distribution of the polymer polyols, or to get equivalent stability using less stabilizer.
In a process for preparing a polymer polyol wherein at least one ethylenically unsaturated monomer is polymerized in the presence of an effective amount of a conventional stabilizer to form a dispersion of particles in the continuous polyol phase and wherein the stabilizer is a partially capped triol, the improvement comprising substituting a more efficient stabilizer for the conventional stabilizer, the more efficient stabilizer being a stabilizer prepared from a formulation including a low molecular weight diol, a coupling agent and a capping agent; and wherein the stabilizer is a polymer having: terminal polymerizable ethylenically unsaturation; an average molecular weight of from about 6,000 to about 100,000; an average hydroxyl functionality of up to about 1.0 hydroxyl groups per molecule; and is present at a from about 0.1 to about 10 percent of the weight of the total polymer polyol.
In addition, the presence of the stabilizer has an effect on the physical properties of polyurethanes made from the polymer polyol. Conventional stabilizers, which often have multiple hydroxyl functionalities would sometimes tend to cause deficiencies in various physical properties, especially fatigue properties. Also, the stabilizer is believed to contribute in some instances to poor compression sets in polyurethane foams.
For these reasons, it would be desirable to provide a stabilizer useful in preparing a polymer polyol, which stabilizer can be used in relatively small quantities and which provides improved stability and/or particle size distribution in the polymer polyol. It would be further desirable to provide a polymer polyol which is useful to prepare polyurethane foams with improved properties relative to polymer polyols made used a conventional stabilizer.